Coated copper particles and method for producing the same

ABSTRACT

A method for producing coated copper particles having a surface coated with an aliphatic carboxylic acid, wherein the method comprises obtaining a reaction mixture containing copper formate, an amino alcohol, an aliphatic carboxylic acid having an aliphatic group having 5 or more carbon atoms, and a solvent, and subjecting a complex compound formed in the reaction mixture to thermal decomposition treatment to form metal copper, wherein a ΔSP value, which is a difference in SP value between the amino alcohol and the solvent, is 4.2 or more.

FIELD OF THE INVENTION

The present invention relates to a coated copper particle and a methodfor producing the same.

BACKGROUND ART

In recent years, in the fields of electronic devices, the technicalfield called printable electronics has attracted attention, in which avery fine wiring of a micrometer size is directly formed by an inkjetprinting method or other printing methods without a need of patterningof a wiring or a protective film by light exposure. Fine particles ofgold or silver were mainly used at the beginning of the development ofthis technique, but gold has a problem of the cost, and silver hasproblems about the electromigration and the corrosion resistance for,for example, corrosion due to a sulfide gas. As a means for solvingthese problems, copper materials have attracted attention. Coppermaterials exhibit high conductivity equivalent to gold or silver and areconsiderably excellent in respect of the electromigration, as comparedto silver, and further have excellent corrosion resistance.

Gold and silver, which are a noble metal, have properties such that theyare relatively unlikely to be oxidized. For this reason, when adispersion of metal fine particles of gold or silver is prepared, it iseasy to maintain the dispersion while preventing an oxide film fromforming on the surface of the metal fine particles contained in thedispersion. In contrast, copper has properties such that it isrelatively likely to be oxidized, and this tendency is furtherremarkable particularly for the copper fine particles having a particlediameter as small as 200 nm or less due to the size effect and specificsurface area. When a dispersion of copper fine particles is prepared,the copper fine particles contained in the dispersion become in a statein a short period of time such that the surface of the copper particlesis covered with an oxide film, and further the oxide film on the surfaceof the copper particles increases in thickness with the passage of time,so that the portion occupying almost all the particle diameter of thecopper fine particles is often converted to the copper oxide film.Further, 200 nm or less copper fine particles are in a state such thatthe surface of the particles is extremely highly active, and, even whensubjected to a method in which heating or calcination is performed in aninert atmosphere of, for example, nitrogen gas or under vacuumconditions, oxidation may proceed due to a very small amount of oxygenpresent in the atmosphere to inhibit sintering of the copper fineparticles with one another. Further, when reducing calcination isperformed using, for example, hydrogen gas at the final stage ofcalcination, the increase of the oxide film during the calcination maycause marked volume shrinkage upon reducing the film, leading to alowering of the calcination density.

Meanwhile, one of the reasons why the technique using metal fineparticles has attracted attention resides in melting point depressiondue to a size effect. Taking gold as an example, the melting pointdepression due to a size effect has been reported as follows. Gold inthe form of a simple substance has a melting point of 1,064° C. Whengold is in the form of particles having a particle diameter of about 2nm, the melting point is reduced to about 300° C. which is a temperatureat which electronic materials and others can be used. However, when goldis in the form of particles having a particle diameter of more than 20nm, almost no melting point depression is recognized. From the above,single-nanometer-sized metal fine particles having a particle diameterof about 2 nm can be satisfactorily expected to suffer melting pointdepression. With respect to the copper fine particles, however, asurface protecting agent for preventing oxidation is indispensable.Considering the specific surface area of the copper fine particles, theamount of the surface protecting agent required for the copper fineparticles is several times or more the volume of copper, and the surfaceprotecting agent in such a large amount causes marked volume shrinkageduring the sintering, making it difficult to obtain a sintered materialhaving a high density. For removing this disadvantage, a method has beenknown in which single-nanometer-sized particles are formed from a metaloxide in a reducing atmosphere at the sintering stage, and sintered at atemperature of about 300 to 400° C. utilizing the melting pointdepression due to a size effect. In addition, a method has been proposedin which, like a flux effect of a solder, an oxide film covering thesurface of fine particles is removed by a fluxing agent, such as anorganic carboxylic acid, so that the reduced metal surface is exposed,followed by sintering (see, for example, Japanese Unexamined PatentPublication No. 2013-047365).

When the copper fine particles are applied to printable electronics, apaste is prepared from the copper fine particles and then supplied.Therefore, copper fine particles exhibiting a monodisperse particlediameter distribution are prepared so as to obtain a copper pastematerial having excellent dispersion stability. With respect to themethod for producing metal fine particles or metal oxide fine particleshaving a uniform particle diameter, several proposals have been made.For example, with respect to a liquid phase synthesis of metal fineparticles, reference is often made to the LaMer model representing therelationship between the solubility of the solute which constitutes ametal nucleus and the time. According to this, when the rate offormation of metal nuclei having a low solubility is too fast, growth ofparticles occurs in accordance with an aggregation mechanism, so thatthe growth of crystal nuclei is disadvantageously unsatisfactory tocause particles in an aggregate form. For solving this problem, a methodof controlling the rate of formation of metal nuclei which are a solutehas been made. For example, by permitting a material needed for thegrowth of particles to be gradually emitted from a reservoir (solid ormetal chelate), the degree of supersaturation of the solution iscontrolled to suppress new nucleation during the growth of particles, sothat the nucleation period and the particle growth period are separatedso as to allow only the nuclei formed early in the initial stage togrow, making it possible to form monodisperse particles. As a method ofselecting a reservoir for supplying the solute during the growth ofparticles, a solid or complex compound having a satisfactorily lowsolubility or dissolution rate is selected.

In connection with the above, a technique in which a complex compoundderived from copper formate is subjected to thermal decomposition toproduce copper fine particles has been known. Copper formate has adecomposition temperature of about 220° C., but copper formate having acomplex structure can be reduced in the decomposition temperature. Forexample, Japanese Unexamined Patent Publication No. 2011-032558 hasproposed a method in which, using a complex compound of an amino alcoholwhich functions as a bidentate ligand, the complex compound is subjectedto thermal decomposition at 100° C. to produce metal fine particles.Japanese Unexamined Patent Publication Nos. 2008-013466 and 2008-031104have proposed a method in which, using a complex compound of analiphatic amine which functions as a monodentate ligand, the complexcompound is subjected to thermal decomposition at 120° C. to producemetal fine particles.

Further, a method has been made in which the metal nuclei incorporatedinto the growing nuclei in a micro-reaction field using a surfactant arerestricted to control the particle diameter. For example, there has beenproposed a method for producing metal or metal oxide fine particles by areversed micelle method in which nanometer-sized water droplets stablydispersed in an organic solvent using a surfactant are used as areaction field (see, for example, Japanese Unexamined Patent PublicationNos. Hei 08-143916 and 2009-082828 and Japanese Patent No. 3900414).

PRIOR ART REFERENCES Patent Documents

-   Patent document 1: Japanese Unexamined Patent Publication No.    2011-032558-   Patent document 2: Japanese Unexamined Patent Publication No.    2008-013466-   Patent document 3: Japanese Unexamined Patent Publication No.    2008-031104-   Patent document 4: Japanese Unexamined Patent Publication No.    2013-047365-   Patent document 5: Japanese Unexamined Patent Publication No. Hei    08-143916-   Patent document 6: Japanese Unexamined Patent Publication No.    2009-082828-   Patent document 7: Japanese Patent No. 3900414

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, in the technique described in Japanese Unexamined PatentPublication No. 2011-032558, the decomposition temperature of thecomplex compound is too low, and therefore heat generated due todecomposition of the complex compound forms a great amount of metalnuclei at an increasing speed, and further the concentration of coppercontained in the reaction mixture is as relatively high as 1.0 to 2.4mol/L, and hence growth of particles is likely to occur in accordancewith an aggregation mechanism to form coarse particles, causing theyield to be lowered.

Further, in the technique described in Japanese Unexamined PatentPublication Nos. 2008-013466 and 2008-031104, the aliphatic amineconstituting the copper formate complex functions also as a dispersionprotecting agent for metal fine particles, and therefore growth ofparticles is unlikely to occur, making it difficult to produce copperparticles having a particle diameter of 20 nm to submicron.

Furthermore, in the technique described in Japanese Unexamined PatentPublication No. 2013-047365, a copper compound in a solid state, such ascopper oxide having a low solubility, is used as a reservoir, and areduction is conducted while partially dissolving the copper compoundwith an organic carboxylic acid, and therefore the rate of formation ofnuclei is restricted, and aggregation is unlikely to occur at the stageof growth of nuclei, as compared to the dissolution system as describedin Japanese Unexamined Patent Publication No. 2011-032558. Aggregationcan be controlled; however, the time during which nuclei are beingformed is long, and the carbon chain of the carboxylic acid coating theparticles is so short that a satisfactory repulsive force between theparticles cannot be obtained, and therefore it is difficult to produceparticles having a uniform particle size distribution, and further it islikely that copper particles having a surface oxidized are formed.

In the reversed micelle method described in Japanese Unexamined PatentPublication Nos. Hei 08-143916 and 2009-082828 and Japanese Patent No.3900414, micelles are stabilized using a large amount of a surfactant,and therefore the size of the micelles is kept constant during thereaction, but the size of the reaction field is restricted, making itdifficult to produce 20 nm or more particles. Further, in the reversedmicelle method, it is difficult to keep high the concentration of thecopper compound in the reaction mixture, and hence the reversed micellemethod is unsuitable for producing a large amount of particles.

In view of the above, an object of the present invention is to solve theproblems accompanying the prior art and to provide a coated copperparticle having both excellent oxidation resistance and excellentsintering property, which have been difficult to achieve in the priorart. Further, an object of the present invention is to provide a methodfor producing coated copper particles in which coated copper particleshaving both excellent oxidation resistance and excellent sinteringproperty can be obtained in a low thermal treatment temperature and alow oxygen-concentration environment.

Means for Solving the Problems

The present inventors have conducted studies with a view toward solvingthe above-mentioned problems. As a result, it has been found that, byappropriately selecting the difference in SP value between the solventand the amino alcohol, which is a complexing agent, contained in thereaction mixture, the reaction system can be constructed so that thesystem is homogeneous at the initial stage of the reaction and forms atwo-phase separation structure at the middle stage of the reaction,making it possible to efficiently produce high-quality coated copperparticles.

The present invention includes the following embodiments.

(1) A method for producing coated copper particles having a surfacecoated with an aliphatic carboxylic acid, wherein the method comprisesobtaining a reaction mixture containing copper formate, an aminoalcohol, an aliphatic carboxylic acid having an aliphatic group having 5or more carbon atoms, and a solvent, and subjecting a complex compoundformed in the reaction mixture to thermal decomposition treatment toform metal copper, wherein a ΔSP value, which is a difference in SPvalue between the amino alcohol and the solvent, is 4.2 or more.

(2) The method for producing coated copper particles according to item(1) above, wherein the amino alcohol has an SP value of 11.0 or more.

(3) The method for producing coated copper particles according to item(1) or (2) above, wherein the temperature for the thermal decompositiontreatment is 100 to 130° C.

(4) The method for producing coated copper particles according to anyone of items (1) to (3) above, wherein the solvent comprises an organicsolvent capable of forming an azeotrope together with water, wherein themethod comprises removing at least part of water formed due to thethermal decomposition treatment in the manner of an azeotropy.

(5) The method for producing coated copper particles according to anyone of items (1) to (4) above, wherein the aliphatic group portion ofthe aliphatic carboxylic acid has 5 to 17 carbon atoms.

(6) The method for producing coated copper particles according to anyone of items (1) to (5) above, wherein the reaction mixture has a copperion concentration of 1.0 to 2.5 mol/liter.

(7) A coated copper particle which is obtained by the method forproducing coated copper particles according to any one of items (1) to(6) above, wherein the coated copper particle has an average primaryparticle diameter D_(SEM) of 0.02 to 0.2 μm, as determined by a SEMexamination, wherein a value of a coefficient of variation of theparticle size distribution (standard deviation SD/average primaryparticle diameter D_(SEM)) is 0.1 to 0.5.

(8) A coated copper particle which is obtained by the method forproducing coated copper particles according to any one of items (1) to(6) above, wherein the coated copper particle has a D_(XRD)/D_(SEM)ratio of 0.25 to 1.00 wherein the D_(XRD)/D_(SEM) ratio is a ratio of acrystal particle diameter D_(XRD), as determined by a powder X-rayanalysis, to an average primary particle diameter D_(SEM), as determinedby a SEM examination.

(9) A conductive composition for screen printing, comprising coatedcopper particles obtained by the method for producing coated copperparticles according to any one of items (1) to (6) above and a medium.

(10) A conductive composition for inkjet printing, comprising coatedcopper particles obtained by the method for producing coated copperparticles according to any one of items (1) to (6) above and a medium.

(11) A circuit formed article comprising a substrate, and a wiringpattern which is disposed on the substrate, and which is a thermaltreatment product of the conductive composition according to item (9) or(10) above.

Effects of the Invention

In the present invention, there can be provided a coated copper particlehaving both excellent oxidation resistance and excellent sinteringproperty, which have been difficult to achieve in the prior art.Further, in the present invention, there can be provided a method forproducing coated copper particles in which coated copper particleshaving both excellent oxidation resistance and excellent sinteringproperty can be obtained in a low thermal treatment temperature and alow oxygen-concentration environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A XRD data of the coated copper particles prepared in Example 1,which is measured immediately after being synthesized.

FIG. 1B XRD data of the coated copper particles prepared in Example 1,which is measured after being stored in the state of a powder in air at25° C. for 4 months.

FIG. 2A Results of a Tof-SIMS analysis made with respect to the coatedcopper particles prepared in Example 1, showing Negative analysisresults.

FIG. 2B Results of a Tof-SIMS analysis made with respect to the coatedcopper particles prepared in Example 1, showing partially enlargedNegative analysis results.

FIG. 3 TG-DTA analysis data of the coated copper particles prepared inExample 1.

FIG. 4 Data obtained by plotting the reaction temperature and the totalamount of the gas generated during the synthesis in Example 1.

FIG. 5 FT-IR analysis data of the distillate distilled during thesynthesis in Example 1.

FIG. 6 XRD data of the coated copper particles prepared in ReferenceExample 2.

FIG. 7 A SEM examination image of the coated copper particles preparedin Example 1.

FIG. 8 A SEM examination image of the coated copper particles preparedin Example 2.

FIG. 9 A SEM examination image of the coated copper particles preparedin Example 4.

FIG. 10 A SEM examination image of the coated copper particles preparedin Comparative Example 1.

FIG. 11 A SEM examination image of the coated copper particles preparedin Comparative Example 2.

FIG. 12 A SEM examination image of the coated copper particles preparedin Comparative Example 3.

FIG. 13 A SEM examination image of the coated copper particles preparedin Comparative Example 4.

FIG. 14 A SEM examination image of the coated copper particles preparedin Comparative Example 5.

FIG. 15 A SEM examination image of the coated copper particles preparedin Example 5.

FIG. 16A A SEM examination image of the coated copper particles preparedin Example 6.

FIG. 16B An enlarged SEM examination image of the coated copperparticles prepared in Example 6.

FIG. 17A A SEM examination image of the coated copper particles preparedin Example 7.

FIG. 17B An enlarged SEM examination image of the coated copperparticles prepared in Example 7.

FIG. 18A A SEM examination image of the coated copper particles preparedin Example 8.

FIG. 18B An enlarged SEM examination image of the coated copperparticles prepared in Example 8.

FIG. 19A A SEM examination image of the coated copper particles preparedin Example 9.

FIG. 19B An enlarged SEM examination image of the coated copperparticles prepared in Example 9.

FIG. 20A An enlarged SEM examination image of the coated copperparticles prepared in Example 1.

FIG. 20B Particle size distribution measurement data of the coatedcopper particles prepared in Example 1.

FIG. 21 XRD data of the coated copper particles prepared in ComparativeExample 6.

FIG. 22 XRD data of the coated copper particles prepared in ComparativeExample 7.

FIG. 23A A SEM examination image of the coated copper particles preparedin Comparative Example 6.

FIG. 23B A SEM examination image of a sintered film which is obtained bypreparing a paste of the coated copper particles prepared in ComparativeExample 6 and calcining the paste in a nitrogen gas atmosphere at 500°C. for one hour.

FIG. 24A A SEM examination image of the coated copper particles preparedin Example 1.

FIG. 24B A SEM examination image of a sintered film which is obtained bypreparing a paste of the coated copper particles prepared in Example 1,i.e., copper paste A and calcining copper paste A in a nitrogen gasatmosphere at 350° C. for one hour.

FIG. 25 A cross-sectional SEM image of a calcined film formed fromcopper paste A.

FIG. 26A A SEM examination image of the coated copper particles preparedin Comparative Example 7.

FIG. 26B A SEM examination image of a sintered film which is obtained bypreparing a paste of the coated copper particles prepared in ComparativeExample 7, i.e., copper paste B and calcining copper paste B in anitrogen gas atmosphere at 350° C. for one hour.

FIG. 27 A cross-sectional SEM image of a calcined film formed fromcopper paste C.

FIG. 28A Diagram showing data for carbon atoms in the XPS uppermostsurface compositional analysis data in Test Example 2.

FIG. 28B Diagram showing data for oxygen atoms in the XPS uppermostsurface compositional analysis data in Test Example 2.

FIG. 28C Diagram showing data for copper atoms in the XPS uppermostsurface compositional analysis data in Test Example 2.

FIG. 29A Diagram showing data for carbon atoms in the XPS uppermostsurface compositional analysis data in Test Example 3.

FIG. 29B Diagram showing data for oxygen atoms in the XPS uppermostsurface compositional analysis data in Test Example 3.

FIG. 29C Diagram showing data for copper atoms in the XPS uppermostsurface compositional analysis data in Test Example 3.

FIG. 30A Diagram showing the distribution of carbon atoms in theXPS-Depth Profile compositional analysis data in Test Example 2.

FIG. 30B Diagram showing the distribution of oxygen atoms in theXPS-Depth Profile compositional analysis data in Test Example 2.

FIG. 30C Diagram showing the distribution of copper atoms in theXPS-Depth Profile compositional analysis data in Test Example 2.

FIG. 31A Diagram showing the distribution of carbon atoms in theXPS-Depth Profile compositional analysis data in Test Example 3.

FIG. 31B Diagram showing the distribution of oxygen atoms in theXPS-Depth Profile compositional analysis data in Test Example 3.

FIG. 31C Diagram showing the distribution of copper atoms in theXPS-Depth Profile compositional analysis data in Test Example 3.

MODE FOR CARRYING OUT THE INVENTION

In the present specification, the term “step” means not only anindependent step but also a combination of steps which cannot bedistinguished from one another as long as a desired purpose of the stepsis achieved. Further, the range of values indicated using thepreposition “to” means a range of values including the respective valuesshown before and after the preposition “to” as the minimum value and themaximum value. Furthermore, with respect to the amount of the componentof a composition, when a plurality of materials corresponding to thecomponents are present in the composition, the amount of the componentsin the composition means the total amount of the materials present inthe composition unless otherwise specified.

<Method for Producing Coated Copper Particles>

The method for producing coated copper particles according to thepresent embodiment is a method for producing coated copper particleshaving a surface coated with an aliphatic carboxylic acid, wherein themethod comprises obtaining a reaction mixture containing copper formate,an amino alcohol, an aliphatic carboxylic acid having an aliphatic grouphaving 5 or more carbon atoms, and a solvent, and subjecting a complexcompound formed in the reaction mixture to thermal decompositiontreatment to form metal copper, wherein a ΔSP value, which is adifference in SP value between the amino alcohol and the solvent, is 4.2or more.

Copper formate is used as a starting material, and a thermaldecomposition reduction reaction of a copper formate complex is allowedto proceed in a liquid phase, and, as the reaction proceeds, an aminoalcohol incompatible with the reaction solvent is emitted from thecopper formate complex into the reaction solvent to form a new reactionfield which is like a water-in-oil emulsion. It is considered that,while continuously generating copper metal nuclei in the reaction field,a nucleus growth reaction proceeds, forming reduced copper particleshaving excellent oxidation resistance and excellent sintering propertyand having a particle diameter controlled so that the particle size isuniform. Further, by appropriately controlling the rate of thermaldecomposition of the copper formate complex, the supply of the solute iscontrolled. This is considered to control the growth of metal nuclei toform reduced copper particles having a more uniform particle size.

When an aliphatic carboxylic acid is further present in the liquidphase, the formed reduced copper particles are coated with the aliphaticcarboxylic acid through physical adsorption at a high density. The thusproduced coated copper particles are comprised of reduced copperparticles having almost no oxide film, and have the surface thereofcoated with the aliphatic carboxylic acid through physical adsorption,and therefore are considered to have excellent balance between theoxidation resistance and the sintering property. By virtue of this, thealiphatic carboxylic acid coating the copper particles as an organicprotecting agent is removed at a temperature of 400° C. or lower in thecalcination step for the coated copper particles. Thus, sintering of thecoated copper particles can be achieved in a low oxygen-concentrationatmosphere which can be created using a means of, for example, purgingwith nitrogen without using a reducing atmosphere of, for example,hydrogen gas. Therefore, the coated copper particles can be effectivelyused in a site to which conventional copper particles that require areducing atmosphere for sintering cannot be applied, for example, a sitewhich poses a problem of change in properties due to hydrogenembrittlement or a reaction with hydrogen. Further, the coated copperparticles can be sintered utilizing an existing facility, such as anitrogen-purged reflow furnace, and therefore are excellent from aneconomical point of view.

The reaction mixture used in the method for producing coated copperparticles according to the present embodiment contains copper formate,at least one amino alcohol, at least one aliphatic carboxylic acidhaving an aliphatic group having 5 or more carbon atoms, and a solvent.The reaction mixture may further contain another additive if necessary.

(Copper Formate)

Copper formate is comprised of a bivalent copper ion and 2 mol of aformic acid ion relative to 1 mol of the copper ion. The copper formatemay be either an anhydride or a hydrate. Further, commercially availablecopper formate may be used, or copper formate newly prepared may beused.

A method in which copper formate is subjected to thermal decompositionto obtain reduced copper fine particles is disclosed in, for example,Japanese Examined Patent Publication No. Sho 61-19682. Formic acid hasreducing properties unlike a general carboxylic acid. Therefore, whencopper formate is subjected to thermal decomposition, it is possible toreduce a bivalent copper ion. For example, it is known that when copperformate anhydride is heated in an inert gas, it suffers thermaldecomposition at 210 to 250° C. to form metal copper.

With respect to the amount of the copper formate contained in thereaction mixture, there is no particular limitation, and the amount ofthe copper formate can be appropriately selected according to, forexample, the purpose. For example, from the viewpoint of the productionefficiency, the amount of the copper formate contained in the reactionmixture is preferably 1.0 to 2.5 mol/liter, more preferably 1.5 to 2.5mol/liter, especially preferably 2.0 to 2.5 mol/liter.

(Amino Alcohol)

With respect to the amino alcohol, there is no particular limitation aslong as it is an alcohol compound having at least one amino group andbeing capable of forming a complex compound together with copperformate. By virtue of the amino alcohol present in the reaction mixture,a complex compound is formed from copper formate, making it possible tosolubilize copper formate in the solvent.

The amino alcohol is preferably a monoaminomonoalcohol compound, morepreferably a monoaminomonoalcohol compound having an unsubstituted aminogroup. Further, the amino alcohol is also preferably amonodentate-coordinating monoaminomonoalcohol compound.

With respect to the boiling point of the amino alcohol, there is noparticular limitation. However, it is preferred that the boiling pointof the amino alcohol is higher than the reaction temperature for thethermal decomposition treatment.

Specifically, the boiling point of the amino alcohol is preferably 120°C. or higher, more preferably 130° C. or higher. The upper limit of theboiling point is not particularly limited, and is, for example, 400° C.or lower, preferably 300° C. or lower.

Further, in view of the polarity, the amino alcohol preferably has an SPvalue of 11.0 or more, more preferably 12.0 or more, further preferably13.0 or more. The upper limit of the SP value of the amino alcohol isnot particularly limited, and is, for example, 18.0 or less, preferably17.0 or less.

In the present specification, the SP value is according to theHildebrand's definition. According to this definition, an SP value meansa square root of intermolecular bonding energy E₁ per 1 mL of a sampleat 25° C. As a calculation method for SP value, the method described in“The Japan Petroleum Institute Homepage”(http://sekiyu-gakkai.or.jp/jp/dictionary/petdiesolvent.html#solubility2)is employed. Specifically, an SP value is calculated as follows.

Intermolecular bonding energy E₁ is a value obtained by subtracting agas energy from an evaporation latent heat. Evaporation latent heat Hbis given by the following equation using boiling point Tb of a sample.

Hb=21×(273+Tb)

From the Hb value, molar evaporation latent heat H₂₅ at 25° C. isdetermined by the following equation.

H ₂₅ =Hb×[1+0.175×(Tb−25)/100]

From molar evaporation latent heat H₂₅, intermolecular bonding energy Eis determined by the following equation.

E=H ₂₅−596

From intermolecular bonding energy E, intermolecular bonding energy E₁per 1 mL of a sample is determined by the following equation.

E ₁ =E×D/Mw

In the above equation, D is a density of the sample, and Mw is amolecular weight of the sample. From E₁, an SP value is determined bythe following equation.

SP=(E ₁)^(1/2)

With respect to the solvent containing an OH group, correction of +1 forone OH group is needed.

[see, for example, Mitsubishi Oil Technical Data, No. 42, p3, p11(1989)]

Specific examples of amino alcohols include 2-aminoethanol (boilingpoint: 170° C.; SP value: 14.54), 3-amino-1-propanol (boiling point:187° C.; SP value: 13.45), 5-amino-1-pentanol (boiling point: 245° C.;SP value: 12.78), DL-1-amino-2-propanol (boiling point: 160° C.; SPvalue: 12.74), and N-methyldiethanolamine (boiling point: 247° C.; SPvalue: 13.26), and preferred is at least one member selected from thegroup consisting of these amino alcohols.

The amino alcohols may be used individually or in combination.

With respect to the amount of the amino alcohol contained in thereaction mixture, there is no particular limitation, and the amount ofthe amino alcohol can be appropriately selected according to, forexample, the purpose. The amount of the amino alcohol contained is, forexample, preferably in the range of from 1.5 to 4.0 times, morepreferably in the range of from 1.5 to 3.0 times the amount of copperions contained in the reaction mixture, in terms of mol. When the amountof the amino alcohol contained is 1.5 times or more the amount of copperions, in terms of mol, satisfactory solubility of copper formate can beobtained, so that the period of time required for the reaction can bereduced. On the other hand, when the amount of the amino alcoholcontained is 4.0 times or less the amount of copper ions, in terms ofmol, contamination of the formed coated copper particles can besuppressed.

(Aliphatic Carboxylic Acid)

With respect to the aliphatic carboxylic acid, there is no particularlimitation as long as it is a long-chain aliphatic carboxylic acidhaving an aliphatic group having 5 or more carbon atoms. The aliphaticgroup may be either linear or branched, and may be either a saturatedaliphatic group or an unsaturated aliphatic group. The aliphatic grouphas 5 or more carbon atoms, preferably 5 to 17 carbon atoms, morepreferably 7 to 17 carbon atoms. When the aliphatic group has 5 or morecarbon atoms, the variation as an index of the particle sizedistribution of the resultant coated copper particles tends to becomesmaller. The reason for this is presumed that, for example, there is ahigh correlation between the length of the carbon chain and the van derWaals force which largely influences force of association. Specifically,it is considered that a carboxylic acid having a long carbon chain hasso strong force of association that it contributes to stabilization of awater-in-oil emulsion-like phase, which is a micro-reaction field,making it possible to efficiently produce copper particles having auniform particle diameter.

It is preferred that the boiling point of the aliphatic carboxylic acidis higher than the temperature for the thermal decomposition treatment.Specifically, the boiling point of the aliphatic carboxylic acid ispreferably 120° C. or higher, more preferably 130° C. or higher. Theupper limit of the boiling point is not particularly limited, and is,for example, 400° C. or lower. When the boiling point of the aliphaticcarboxylic acid is 400° C. or lower, the resultant coated copperparticles tend to be further improved in the sintering property.

Specific examples of aliphatic carboxylic acids include oleic acid,linoleic acid, stearic acid, heptadecanoic acid, lauric acid, andoctanoic acid, and preferred is at least one member selected from thegroup consisting of these aliphatic carboxylic acids.

The aliphatic carboxylic acids may be used individually or incombination.

With respect to the amount of the aliphatic carboxylic acid contained inthe reaction mixture, there is no particular limitation, and the amountof the aliphatic carboxylic acid can be appropriately selected accordingto, for example, the purpose.

The amount of the aliphatic carboxylic acid contained is, for example,based on the mol of copper ions contained in the reaction mixture,preferably in the range of from 2.5 to 25 mol %, more preferably in therange of from 5.0 to 15 mol %. When the amount of the aliphaticcarboxylic acid contained is 25 mol % or less, based on the mol ofcopper ions, it is likely that an increase of the viscosity of thereaction system can be suppressed. On the other hand, when the amount ofthe aliphatic carboxylic acid contained is 2.5 mol % or more, based onthe mol of copper ions, it is likely that a satisfactory reaction ratecan be obtained to improve the productivity, and the variation as anindex of the particle size distribution tends to become smaller.

(Solvent)

With respect to the solvent constituting the reaction mixture, there isno particular limitation as long as the solvent is selected so that itdoes not excessively inhibit the reduction reaction caused by formicacid and a ΔSP value, which is a difference in SP value between theamino alcohol and the solvent, becomes 4.2 or more, and the solvent canbe appropriately selected from organic solvents generally used.

When the ΔSP value, which is a difference between the SP value of theamino alcohol and the SP value of the solvent, is 4.2 or more, there canbe obtained coated copper particles having a particle size distributionhaving a narrow width such that the particle diameter of the formedcoated copper particles is uniform.

The ΔSP value is 4.2 or more, and, from the viewpoint of the formationof a reaction field and the quality of the coated copper particles, theΔSP value is preferably 4.5 or more, more preferably 5.0 or more. Theupper limit of the ΔSP value is not particularly limited, and, forexample, the ΔSP value is 11.0 or less, preferably 10.0 or less.

The SP value of the solvent is selected so that the ΔSP value becomes4.2 or more. However, it is preferred that the SP value of the solventis smaller than the SP value of the amino alcohol. The solventpreferably has an SP value of 11.0 or less, more preferably 10.0 orless. The lower limit of the SP value of the solvent is not particularlylimited, and, for example, the SP value of the solvent is preferably 7.0or more.

Further, it is preferred that the boiling point of the solvent is higherthan the temperature for the thermal decomposition treatment.Specifically, the boiling point of the solvent is preferably 120° C. orhigher, more preferably 130° C. or higher. The upper limit of theboiling point is not particularly limited, and, for example, the boilingpoint of the solvent is 400° C. or lower, preferably 300° C. or lower.

Further, it is preferred that the solvent is an organic solvent capableof forming an azeotrope together with water. When an azeotrope of thesolvent with water can be formed, water formed in the reaction mixturecan be easily removed from the reaction system by the thermaldecomposition treatment.

Specific examples of solvents include ethylcyclohexane (boiling point132° C.; SP value: 8.18), C9 cyclohexane [trade name: SWACLEAN #150,manufactured by Maruzen Petrochemical Co., Ltd.] (boiling point: 149°C.; SP value: 7.99), and n-octane (boiling point: 125° C.; SP value:7.54), and preferred is at least one member selected from the groupconsisting of these solvents.

The solvents may be used individually or in combination.

When two or more solvents are used in combination, it is preferred thata prime solvent incompatible with the amino alcohol and a co-solventcompatible with the amino alcohol are contained. Specific examples ofprime solvents are those mentioned above for the solvents.

A preferred boiling point of the co-solvent is similar to that of theprime solvent. The SP value of the co-solvent is preferably larger thanthat of the prime solvent, and is more preferably large such that theco-solvent is compatible with the amino alcohol. Specific examples ofco-solvents include glycol ethers, such as EO glycol ethers, PO glycolethers, and dialkyl glycol ethers. More specific examples include EOglycol ethers, such as methyl diglycol, isopropyl glycol, and butylglycol; PO glycol ethers, such as methylpropylene diglycol,methylpropylene triglycol, propylpropylene glycol, and butylpropyleneglycol; and dialkyl glycol ethers, such as dimethyl diglycol, andpreferred is at least one member selected from the group consisting ofthese co-solvents. Any of these co-solvents are available from, forexample, Nippon Nyukazai Co., Ltd.

When two or more solvents are used in combination, an SP value of thesolvents is determined as an average SP value by calculation consideringthe SP values and molar volumes of the individual solvents contained.Specifically, when solvent 1 and solvent 2 are used in combination, anaverage SP value is calculated by the following equation.

δ₃ =[V ₁×δ₁ +V ₂×δ₂]/(V ₁ +V ₂)

δ₃: average SP value of the mixed solvent;δ₁: SP value of solvent 1; V₁: molar volume of solvent 1;δ₂: SP value of solvent 2; V₂: molar volume of solvent 2.

The amount of the solvent contained in the reaction mixture ispreferably selected so that the copper ion concentration of the reactionmixture becomes 1.0 to 2.5 mol/liter, more preferably 1.5 to 2.5mol/liter. When the copper ion concentration of the reaction mixture is1.0 mol/liter or more, the productivity is further improved. When thecopper ion concentration of the reaction mixture is 2.5 mol/liter orless, an increase of the viscosity of the reaction mixture can besuppressed, achieving excellent stirring properties.

(Complex Compound)

From the reaction mixture containing copper formate, an amino alcohol, along-chain aliphatic carboxylic acid, and a solvent, a complex compoundderived from copper formate is formed. With respect to the structure ofthe complex compound, there is no particular limitation, and the complexcompound may contain only one type of structure or two types ofstructures. Further, the complex compound may be changed in theconstruction as the thermal decomposition treatment proceeds. That is,the complex compound mainly present at the initial stage of the thermaldecomposition treatment and the complex compound mainly present at thelast stage of the thermal decomposition treatment may have differentconstructions from each other.

The complex compound formed in the reaction mixture preferably containsa copper ion, and a formic acid ion and an amino alcohol as ligands. Byvirtue of containing an amino alcohol as a ligand, the thermaldecomposition temperature of the complex compound is lowered.

Examples of the complex compounds formed in the reaction mixtureinclude, specifically, a complex compound having one copper ion to whichtwo molecules of formic acid ions and two molecules of amino alcoholcoordinate, and a complex compound having one copper ion to which onemolecule of formic acid ion, one molecule of aliphatic carboxylic acid,and two molecules of amino alcohol coordinate.

The complex compound formed in the reaction mixture forms metal copperby a thermal decomposition treatment. The temperature for the thermaldecomposition treatment may be appropriately selected according to, forexample, the structure of the complex compound. Generally, the thermaldecomposition temperature of copper formate is about 220° C. However,when copper formate and an amino alcohol together form a complexcompound, the thermal decomposition temperature of the compound isconsidered to be lowered to about 110 to 120° C., for example, asdescribed in Japanese Unexamined Patent Publication No. 2008-013466.Therefore, the temperature for the thermal decomposition treatment ispreferably 100 to 130° C., more preferably 110 to 130° C. When thetemperature for the thermal decomposition treatment is 130° C. or lower,it is likely that formation of an acid amide due to a dehydrationreaction of the aliphatic carboxylic acid and the amino alcohol issuppressed, so that the washing property for the obtained coated copperparticles is improved.

Thermal decomposition of the complex compound forms metal copper, andthe aliphatic carboxylic acid present in the reaction mixture adsorbsonto the surface of the formed metal copper, making it possible toobtain coated copper particles having a surface coated with thealiphatic carboxylic acid. The adsorption of the aliphatic carboxylicacid onto the surface of the metal copper is preferably physicaladsorption. By virtue of this, the sintering property of the coatedcopper particles is further improved. Physical adsorption of thealiphatic carboxylic acid is promoted by suppressing the formation ofcopper oxide in the thermal decomposition of the complex compound.

In the thermal decomposition treatment, it is preferred that at leastpart of water formed due to the thermal decomposition reaction of thecomplex compound is removed. By removing water in the thermaldecomposition treatment, the formation of copper oxide can be moreefficiently suppressed.

With respect to the method for removing water, there is no particularlimitation, and the method can be appropriately selected from methodsgenerally used for removing water. For example, it is preferred that theformed water is removed in the manner of an azeotropy using, as asolvent, an organic solvent capable of forming an azeotrope togetherwith water.

The time for the thermal decomposition treatment may be appropriatelyselected according to, for example, the temperature for the thermaldecomposition treatment. For example, the time for the thermaldecomposition treatment can be 30 to 180 minutes. Further, theatmosphere for the thermal decomposition treatment is preferably aninert atmosphere, such as a nitrogen gas atmosphere.

In the method for producing coated copper particles, as examples offactors controlling the particle size distribution of the coated copperparticles formed, there can be mentioned the type and amount of thealiphatic carboxylic acid added, the concentration of the copper formatecomplex, and the ratio for the mixed solvent (prime solvent/co-solvent).The factor controlling the size of the coated copper particles can bemade constant by appropriately maintaining the temperature increase ratedetermining the number of the metal nuclei generated, i.e., the amountof heat introduced into the reaction system, and the stirring speedrelated to the size of the micro-reaction field.

The method for producing coated copper particles is advantageous inthat, by performing an easy operation of preparing a reaction mixturecontaining copper formate, an amino alcohol, an aliphatic carboxylicacid, and a solvent and subjecting the reaction mixture to thermaldecomposition treatment at a desired temperature, coated copperparticles having a uniform particle diameter and having excellentoxidation resistance and excellent sintering property can be efficientlyproduced.

In the method for producing coated copper particles, coated copperparticles having a narrow particle size distribution are obtained. Thereason for this can be considered, for example, as follows.

Specifically, a difference in SP value between the amino alcohol, whichis a complexing agent for solubilizing copper formate in the reactionsolvent, and the solvent, i.e., a ΔSP value is 4.2 or more, andtherefore copper formate is dissolved in the state of a copperformate-amino alcohol complex, or the copper formate-amino alcoholcomplex having one molecule of formic acid replaced by an aliphaticcarboxylic acid, but, when the complex undergoes thermal decompositionto liberate the amino alcohol as a complexing agent, the liberated aminoalcohol is incompatible with the solvent and starts to form two phases.The liberated amino alcohol has a high affinity with copper formate andthe copper formate-amino alcohol complex, and therefore serves as a newcomplexing agent or solvent for copper formate to form an inner nucleus(droplet) having a high polarity, so that the solvent having a lowpolarity surrounds the outer surface of the inner nucleus to form atwo-phase structure that is like a water-in-oil emulsion, which ispresumed to function as a micro-reaction field.

Further, the water in the reaction system and the formic acid eliminateddue to replacement by the aliphatic carboxylic acid are present in themicro-reaction field. A reaction proceeds in a state such that the metalnuclei, particles grown from the metal nuclei, the copper formate-aminoalcohol complex which is a source of generation of the metal nuclei, thecopper formate-amino alcohol complex having one molecule of formic acidreplaced by the aliphatic carboxylic acid, water, and formic acid areisolated in the micro-reaction field. As the aliphatic carboxylic acidis fixed as a coating material for the metal copper grown particles andreduced, the thermal decomposition mechanism for the copper formatecomplex proceeds according to the below-mentioned reaction formulae 1 to3 at the initial stage of the reaction, and subsequently the mechanismof reaction formula 4 proceeds so that the gas component generated ischanged. In the micro-reaction field, CuO is formed due to a hydrolysisof the copper formate-amino alcohol complex by water shown in reactionformula 5, but is reduced again through a reaction of reaction formula 6or reaction formula 7, making it possible to produce reduced copperparticles free of copper(I) oxide and copper(II) oxide. Further, thenumber of copper atoms contained in the micro-reaction field is limited,and therefore the particle diameter of the formed copper particles iscontrolled to be constant.

It is considered that copper particles having no copper oxide formed onthe surface thereof are formed in the micro-reaction field, andtherefore the aliphatic carboxylic acid present in the micro-reactionfield is likely to physically adsorb on the copper particles, so thatcoated copper particles having a uniform particle diameter and havingexcellent oxidation resistance and excellent sintering property can beefficiently obtained.

The method for producing coated copper particles may further have, forexample, a washing step, a separating step, and a drying step for theobtained coated copper particles after the thermal decompositiontreatment. As an example of a washing step for the coated copperparticles, there can be mentioned a washing step using an organicsolvent. Examples of organic solvents used in the washing step includealcohol solvents, such as methanol, and ketone solvents, such asacetone. These organic solvents may be used individually or incombination.

<Coated Copper Particle>

The coated copper particle according to the present embodiment isproduced by the above-described method for producing coated copperparticles, and has an average primary particle diameter D_(SEM) of 0.02to 0.2 μm, as determined by a SEM examination, wherein a value of acoefficient of variation of the particle size distribution (standarddeviation SD/average primary particle diameter D_(SEM)) is 0.1 to 0.5.

The coated copper particle is produced by the above-described method forproducing coated copper particles, and therefore has a small coefficientof variation of the particle size distribution such that the particlediameter is uniform. The coated copper particles having a smallcoefficient of variation of the particle size distribution can achievean effect that a dispersion of the coated copper particles havingexcellent dispersibility and having a high concentration can be easilyprepared.

The coated copper particle according to the present embodiment isobtained by the above-described method for producing coated copperparticles, and has a D_(XRD)/D_(SEM) ratio of 0.25 to 1.00 wherein theD_(XRD)/D_(SEM) ratio is a ratio of a crystal particle diameter D_(XRD),as determined by a powder X-ray analysis, to an average primary particlediameter D_(SEM), as determined by a SEM examination. The coated copperparticle is produced by the above-described method for producing coatedcopper particles, and therefore can be reduced in a difference betweenthe crystal particle diameter and the average primary particle diameter.Accordingly, an effect can be obtained such that the coated copperparticle exhibits excellent oxidation resistance, and consequently, isfurther improved in the sintering property.

The coated copper particle according to the present embodiment isobtained by the above-described method for producing coated copperparticles, and therefore the surface of the copper particle is coatedwith an aliphatic carboxylic acid. The aliphatic carboxylic acid coatingthe copper particle is a coating material which is localized on thesurface of the copper particles to suppress oxidation or aggregation ofthe copper particles, and is removed from the surface of the particlesduring the sintering, and further decomposes or volatilizes at thesintering temperature or lower, and hence is prevented from remaining inthe copper film formed by sintering. The reason for this is presumedthat, for example, the aliphatic carboxylic acid physically adsorbs onthe surface of the copper particles. Further, the copper particlesconstituting the coated copper particles have a uniform particlediameter, and therefore have excellent dispersibility. Furthermore, adifference between the diameter of the crystal particles constitutingthe copper particles and the diameter determined by a SEM examination issmall, and therefore a plurality of the copper particles in an aggregateform do not constitute the coated copper particles, and inhibition ofsintering caused due to the coating material, impurities, oxide layerand others present at boundary portions of the particles in an aggregateform is prevented.

<Conductive Composition>

The conductive composition according to the present embodiment comprisesat least one type of the coated copper particles obtained by theabove-described method for producing coated copper particles, and amedium. The conductive composition can be advantageously used in forminga wiring pattern, and a wiring pattern having excellent conductivity canbe easily formed at low temperatures from the conductive composition.

That is, the present embodiment encompasses the use of theabove-mentioned coated copper particles as a conductive composition.

The constitution of the medium contained in the conductive compositioncan be appropriately selected according to, for example, the purpose ofthe conductive composition.

For example, when the conductive composition is for use in screenprinting, examples of media include hydrocarbon solvents, higher alcoholsolvents, cellosolve, and cellosolve acetate solvents.

The solids content of the conductive composition for screen printing canbe, for example, 40 to 95% by mass. The term “solids” of the conductivecomposition means a total amount of nonvolatile components.

Further, for example, when the conductive composition is for use ininkjet printing, examples of media include hydrocarbon solvents, higheralcohol solvents, cellosolve, and cellosolve acetate solvents.

The solids content of the conductive composition for inkjet printing canbe, for example, 40 to 90% by mass.

The conductive composition, if necessary, can further contain anotheradditive, in addition to the coated copper particles and medium.Examples of other additives include coupling agents, such as a silanecoupling agent and a titanate coupling agent, and dispersants, such as apolyester dispersant and a polyacrylic acid dispersant.

<Circuit Formed Article>

The circuit formed article according to the present embodiment comprisesa substrate, and a wiring pattern which is disposed on the substrate,and which is a thermal treatment product of the above-mentionedconductive composition. The wiring pattern is formed from theabove-mentioned conductive composition, and therefore the conductivityof the wiring pattern is excellent. Further, the wiring pattern can beformed at low temperatures, and hence the freedom of selection of thesubstrate is large.

Examples of materials for the substrate include a polyimide film, glass,a ceramic, and a metal. With respect to the thickness of the substrate,there is no particular limitation, and the thickness of the substratecan be appropriately selected according to, for example, the purpose.The thickness of the substrate can be, for example, 0.01 to S mm.

The formation of a wiring pattern can be made by, for example, applyingthe conductive composition onto a substrate so as to form a desiredpattern, and subjecting the applied conductive composition to thermaltreatment. By virtue of using the conductive composition, a wiringpattern having a desired pattern and having excellent conductivity canbe efficiently formed at low temperatures.

The circuit formed article can be produced by a method for producing acircuit formed article, which comprises, for example, the steps ofproviding a substrate, applying the conductive composition onto asubstrate, and subjecting the applied conductive composition to thermaltreatment. That is, the present embodiment encompasses a method forproducing a circuit formed article using the above-mentioned conductivecomposition.

With respect to the method for applying the conductive composition,there is no particular limitation, and the conductive composition can beapplied by, for example, an inkjet printing method, a screen printingmethod, a flexographic printing method, or a dispense method. The amountof the conductive composition applied can be appropriately selectedaccording to, for example, the purpose, and, for example, can beselected so that the thickness of the conductive composition obtainedafter the thermal treatment becomes 1 to 100 μm.

The temperature for the thermal treatment of the conductive compositioncan be, for example, 200 to 600° C., preferably 250 to 450° C.

The time for the thermal treatment can be, for example, 1 to 120minutes, preferably 5 to 60 minutes.

The atmosphere for the thermal treatment is preferably a lowoxygen-concentration atmosphere. Examples of low oxygen-concentrationatmospheres include a nitrogen gas atmosphere and an argon gasatmosphere. Further, the oxygen concentration of the atmosphere for thethermal treatment is preferably 1,000 ppm or less.

EXAMPLES

Hereinbelow, the present invention will be described in more detail withreference to the following Examples, which should not be construed aslimiting the scope of the present invention.

Details of the test methods used in the following Examples are shownbelow.

<Calculation of an Average Primary Particle Diameter and a Variation>

Measurement apparatus: FE-EPMA JXA-8510F, manufactured by JEOL LTD.Average primary particle diameter: Average value for 20 samplesVariation: Value calculated from (standard deviation/average value) for20 samples

<SEM Examination Image>

Measurement apparatus: FE-EPMA JXA-8510F, manufactured by JEOL LTD.

Conditions for Measurement:

Accelerating voltage: 6 KV or 15 KV

Magnification for examination: ×10,000 to ×75,000

<Measurement of Powder X-Ray Diffraction (XRD)>

Measurement apparatus: XRD-6100, manufactured by Shimadzu Corporation

Conditions for Measurement:

Target: Cu

Tube voltage: 40 KV; Tube current: 30.0 mA

<Measurement of Tof-SIMS (Time-of-Flight Secondary Ion MassSpectrometer)>

Measurement apparatus: PHI TRIFT IV type, manufactured by ULVAC-PHI,Inc.

Conditions for Measurement:

Primary ion species: Au

Accelerating voltage: 30 KV

<TG-DTA Measurement>

Measurement of an organic residue content and a metal contentMeasurement apparatus: TG8120, manufactured by Rigaku CorporationTemperature increase rate: 10° C./minTemperature range for measurement: 25 to 600° C.Atmosphere for measurement: Nitrogen, 100 ml/min

<Particle Size Distribution Measurement>

Measurement of Laser Diffraction Scattering-Type Particle SizeDistribution

Measurement apparatus: LA-960, manufactured by Horiba, Ltd.Solvent for measurement: KYOWANOL MDispersant: Polyacrylic acid dispersantDispersing method: Ultrasonic waves, 5 minutes

<Electric Resistance Value Measurement>

Measurement of a Volume Resistivity Value

Measurement apparatus: K-705RS, manufactured by Kyowa Riken Co., Ltd.Measurement method: Four-terminal measurement methodNumber of points measured: Average for n=5Thickness of a conductive film: Determined by examination of across-section under a SEM

<Uppermost Surface Compositional Analysis and Depth Profile Analysis byXPS>

Measurement apparatus: JPS-9010MX, manufactured by JEOL LTD.High-speed etching ion gun: XP-HSIG3

Conditions for Depth Profile Analysis

Ion beam diameter: φ 15 mmAr ion accelerating voltage: 500 V (current: 8.6 mA), which correspondsto a SiO etching rate of 20 to 25 nm/min(The direction of from Data_0 to Data_6 corresponds to the direction offrom the bottom to the top.)Data_0: No etchingData_1: Execution etching time: 0.9 second (total: 0.9 second)Data_2: Execution etching time: 3.0 seconds (total: 3.9 seconds)Data_3: Execution etching time: 3.0 seconds (total: 6.9 seconds)Data_4: Execution etching time: 3.0 seconds (total: 9.9 seconds)Data_5: Execution etching time: 3.0 seconds (total: 12.9 seconds)Data_6: Execution etching time: 3.0 seconds (total: 15.9 seconds)

Reference Example 1

An example of the production of the copper formate and copper formateanhydride used in the present Examples is shown below, but a pluralityof methods for producing copper formate have been known, and copperformate produced by the other method may be used.

[Pretreatment for Basic Copper Carbonate]

When basic copper carbonate contains a portion in an aggregate form,such a portion is likely to remain unreacted. Therefore, the basiccopper carbonate was treated using an about 28 mesh sieve.

[Procedure for Synthesis]

0.96 kg of formic acid and 1.44 kg of ion-exchanged water were placed ina 5-liter four-neck flask and, while uniformly stirring the resultantmixture, basic copper carbonate was added portion by portion to themixture. All of the basic copper carbonate was added while taking careof the generation of carbon dioxide gas. After completion of theaddition, the temperature of the mixture was increased to 60° C. toeffect a reaction for 0.5 hour. At a point in time when almost no carbondioxide gas flown out of the flask (which is checked by introducing thedrain to a water trap) was confirmed, part of the copper formate andbasic copper carbonate remained undissolved. 1.60 kg of ion-exchangedwater was further added to the resultant mixture to effect a reaction at60° C. for another 1.0 hour. After confirming that the reaction mixturewas deep blue and transparent, the reaction was terminated, and thereaction mixture was subjected to vacuum evaporation using an evaporatorto remove 1.5 liters of water. At that time, crystals had already beendeposited, so that the mixture was in a slurry form.

The mixture was cooled to room temperature, and the reaction product wasseparated by filtration, and washed with one liter of acetone. Theobtained crystals were colored with greenish blue.

Then, drying dehydration was conducted as follows. The dryingdehydration was performed at a drying temperature of 80° C. or lower(temperature of the powder) in a vacuum at 0.5 KPa (finally). The dryingdehydration caused the crystals to be light blue.

Thermal decomposition temperature of copper formate: 214.9° C. (innitrogen), about 200° C. in air

[Quality Check]

The TG-DTA measurement was made to confirm that the contained Cu %approximated to the theoretical value.

Formula weight of copper formate anhydride: 153.84Contained Cu %=41.3%, Loss %=about 58.5%

Example 1

A 3,000 mL four-neck glass flask equipped with a stirrer, a thermometer,a reflux condenser, a 75 mL Dean-Stark tube, and a nitrogen introducingtube was set in an oil bath. 484 g (3.1 mol) of copper formateanhydride, 68.1 g of lauric acid (manufactured by Kanto Chemical Co.,Inc.) (0.11 equivalent/copper formate anhydride), 150 g of tripropyleneglycol monomethyl ether as a reaction solvent (manufactured by TokyoChemical Industry Co., Ltd.) (0.23 equivalent/copper formate anhydride),and 562 g of SWACLEAN 150 (manufactured by Godo Co., Ltd.) (1.42equivalent/copper formate anhydride) were placed in the flask, and mixedwith one another while stirring at 200 rpm. The resultant mixture washeated in a nitrogen gas atmosphere while stirring at 200 rpm until thetemperature of the mixture became 50° C. To the mixture was addeddropwise slowly 712 g of 3-amino-1-propanol (manufactured by TokyoChemical Industry Co., Ltd.) (3.00 equivalents/copper formateanhydride). After completion of the addition, the resultant mixture washeated while stirring at 340 rpm until the temperature of the mixturebecame about 120° C. The aqueous layer trapped by the Dean-Stark tubewas removed with appropriate timing so as not to be returned to thereaction system. As the temperature increased, the reaction mixturestarted changing in color from deep blue to brown and bubbles of carbondioxide gas were generated. A time when the generation of bubbles ofcarbon dioxide gas was ended was determined as a reaction end point, andthe temperature control using an oil bath was stopped, so that thereaction mixture was cooled to room temperature.

After cooling to room temperature, 550 g of methanol (manufactured byKanto Chemical Co., Inc.) was added to and mixed with the reactionmixture. The resultant mixture was allowed to stand for 30 minutes orlonger, and the resultant supernatant was removed by decantation toobtain a precipitate. To the precipitate were added 550 g of methanol(manufactured by Kanto Chemical Co., Inc.) and 300 g of acetone(manufactured by Kanto Chemical Co., Inc.), followed by mixing with oneanother. The resultant mixture was allowed to stand for 30 minutes orlonger, and the resultant supernatant was removed by decantation toobtain a precipitate, and this operation was repeated once. Theresultant precipitate was transferred to a 500 mL eggplant-shaped flaskwhile washing the precipitate using 550 g of methanol (manufactured byKanto Chemical Co., Inc.). After allowed to stand for 30 minutes orlonger, the resultant supernatant was removed by decantation, and theobtained precipitate was set to a rotary evaporator and subjected tovacuum drying at 40° C. under 1 kPa or less. After completion of thevacuum drying, the precipitate was cooled to room temperature, and thereduced pressure was increased by introducing nitrogen gas, obtaining194 g of brown coated copper particles.

A SEM examination image of the obtained coated copper particles is shownin FIG. 7. Further, an enlarged SEM examination image is shown in FIG.20A, and a particle size distribution is shown in FIG. 20B.

Example 2

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed toDL-1-amino-2-propanol.

A SEM examination image of the obtained coated copper particles is shownin FIG. 8.

Example 3

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed to5-amino-1-pentanol, and that the reaction solvent was changed ton-octane.

Example 4

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed toDL-1-amino-2-propanol, and that the reaction solvent was changed ton-octane.

A SEM examination image of the obtained coated copper particles is shownin FIG. 9.

Comparative Example 1

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed to1-hexylamine.

A SEM examination image of the obtained coated copper particles is shownin FIG. 10.

Comparative Example 2

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed to2-diethylaminoethanol.

A SEM examination image of the obtained coated copper particles is shownin FIG. 11.

Comparative Example 3

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed to2-dimethylaminoethanol.

A SEM examination image of the obtained coated copper particles is shownin FIG. 12.

Comparative Example 4

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that 3-amino-1-propanol was changed to5-amino-1-pentanol.

A SEM examination image of the obtained coated copper particles is shownin FIG. 13.

Comparative Example 5

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that the reaction solvent was changed ton-octanol.

A SEM examination image of the obtained coated copper particles is shownin FIG. 14.

Examples in which the type and amount of the long-chain aliphaticcarboxylic acid were changed are shown below.

Example 5

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that lauric acid was changed to 68.16 g ofoleic acid, that a co-solvent was not used as a solvent, and that theamount of SWACLEAN #150 was changed to 712 g.

A SEM examination image of the obtained coated copper particles is shownin FIG. 15.

Example 6

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that the amount of lauric acid was changedfrom 48 g to 16 g.

SEM examination images of the obtained coated copper particles are shownin FIGS. 16A and 16B.

Example 7

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that the amount of lauric acid was changedfrom 48 g to 144 g.

SEM examination images of the obtained coated copper particles are shownin FIGS. 17A and 17B.

Example 8

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that the reaction solvent was changed to150 g of SWACLEAN #150 and 562 g of methylpropylene triglycol.

SEM examination images of the obtained coated copper particles are shownin FIGS. 18A and 18B.

Example 9

Coated copper particles were synthesized in substantially the samemanner as in Example 1 except that lauric acid was changed to octanoicacid.

SEM examination images of the obtained coated copper particles are shownin FIGS. 19A and 19B.

Comparative Example 6

Coated copper particles were synthesized in accordance with the methoddescribed in Example 1 of Japanese Unexamined Patent Publication No.2013-047365. Specifically, coated copper particles were synthesizedusing acetic acid as a coating material as described below.

14.3 g (0.1 mol) of copper(I) oxide (manufactured by Furukawa ChemicalsCo., Ltd.; particle diameter: 2 to 4μ) as a copper compound, 3.0 g (50mmol) of acetic acid as a coating material, 5.0 g (0.1 mol) of hydrazinehydrate (manufactured by Wako Pure Chemical Industries, Ltd.) as areducing agent, and 100 ml of isopropanol as a solvent were mixed andplaced in a 300 ml four-neck flask. The flask was equipped with acondenser, a thermometer, a nitrogen introducing tube, and a stirrer.The temperature of the resultant mixture was increased to 70° C. whilestirring and passing nitrogen gas at 200 ml/min through the flask, andheating and stirring were continued for one hour to reduce copper(I)oxide, obtaining a coated copper particle dispersion.

The obtained coated copper particle dispersion was subjected tofiltration under a reduced pressure using Kiriyama filter paper No. 5Bto separate a powder. The separated powder was washed with methanol(manufactured by Kanto Chemical Co., Inc.) three times, and subjected tovacuum drying at 40° C. under 1 kPa or less, and cooled to roomtemperature, and then removed after introducing nitrogen gas into thevacuum, obtaining 12 g of a brown powder.

With respect to the obtained powder, an XRD measurement was conducted(shown in FIG. 21). As a result, copper(I) oxide believed to be derivedfrom the raw material was slightly detected.

A SEM examination image of the obtained coated copper particles is shownin FIG. 23A.

Comparative Example 7

Coated copper particles were synthesized by a method in whichComparative Example 6 is scaled up and the reaction time is increasedtwice. 71.5 g (0.5 mol) of copper(I) oxide (manufactured by FurukawaChemicals Co., Ltd.) as a copper compound, 15.0 g (250 mmol) of aceticacid as a coating material, 25.0 g (0.5 mol) of hydrazine hydrate(manufactured by Wako Pure Chemical Industries, Ltd.) as a reducingagent, and 500 ml of isopropanol as a solvent were mixed and placed in a1,000 ml four-neck flask. The flask was equipped with a condenser, athermometer, a nitrogen introducing tube, and a stirrer. The temperatureof the resultant mixture was increased to 70° C. while stirring andpassing nitrogen gas at 200 ml/min through the flask, and heating andstirring were continued for 2 hours to reduce copper(I) oxide, obtaininga coated copper particle dispersion.

The obtained coated copper particle dispersion was subjected tofiltration under a reduced pressure using Kiriyama filter paper No. 5Bto separate a powder. The separated powder was washed with methanol(manufactured by Kanto Chemical Co., Inc.) three times, and subjected tovacuum drying at 40° C. under 1 kPa or less, and cooled to roomtemperature, and then removed after introducing nitrogen gas into thevacuum, obtaining 62 g of a brown powder.

With respect to the obtained powder, an XRD measurement was conducted(shown in FIG. 22). As a result, it was found that copper(I) oxide as araw material was stoichiometrically converted to reduced copper.

A SEM examination image of the obtained coated copper particles is shownin FIG. 26A.

<Evaluation>

For clarifying the composition of the coated copper particles coatedwith an aliphatic carboxylic acid, a powder X-ray analysis, a SEMexamination, a Tof-SIMS surface analysis, and a TG-DTA measurement wereconducted using the coated copper particles produced in Example 1.

For examining the structure of the nucleus and the particle diameter ofthe coated copper particles produced in Example 1, a powder X-rayanalysis was conducted. As seen from the results of the powder X-rayanalysis (FIG. 1A), a peak derived from reduced copper (2θ=around 43.3°)was detected, and copper oxide (2θ=35.5° and 38.7°) and copper(I) oxide(2θ=around 37.0°) were not detected. From this result, it is found thatthe coated copper particle according to the present embodiment has nooxide layer and has a nucleus formed from reduced copper.

A crystal particle diameter was determined by making a calculation usingthe Scherrer's equation from an angle of diffraction of powder X-ray anda half band width. The Scherrer's equation is represented by thefollowing formula (1).

D=Kλ/(β cos θ)  (1)

In the above formula, D is a crystal particle diameter, K is aScherrer's constant (K=1 is substituted on the assumption that thecrystal is a sphere), λ is a wavelength of the X-ray for measurement(CuKα: 1.5418 Å), and β is represented by the following formula (2).

β=b−B  (2)

In the above formula, b is a half band width of the peak, and B is acorrection factor for the apparatus (B=0.114).

From the results of the calculation, crystal particle diameter D_(XRD)of the coated copper particles was found to be 48.9 nm. Average primaryparticle diameter D_(SEM) calculated from the results of the SEMexamination is 85.8 nm, and therefore a D_(XRD)/D_(SEM) ratio calculatedis 0.57, and it is found that the ratio of the crystal particle diameterto the average primary particle diameter is relatively large.

For examining the surface composition of the coated copper particles, aTof-SIMS surface analysis was conducted. As seen from the results of theTof-SIMS surface analysis, free lauric acid almost in agreement with astoichiometrically expected amount was detected (shown in FIG. 2A), and,although in a slight amount, lauric acid bonded to hydroxides of ⁶³Cuand ⁶⁵Cu was also detected (shown in FIG. 2B). From the fact that lauricacid bonded to ⁶³Cu and ⁶⁵Cu was not detected, it has been found thatalmost all the material present on the surface of the coated copperparticles is lauric acid coating the surface through physicaladsorption.

For determining the amount of the organic component coating the surfaceof the coated copper particles, a TG-DTA analysis was conducted (FIG.3). From the results of the TG-DTA analysis, it is found that a heatingloss is 1.09% by mass, and that almost all the organic component iseliminated at around the boiling point of lauric acid. This result alsosuggests that lauric acid physically adsorbs onto the copper particles,and it is expected that the coated copper particles can exhibit alow-temperature sintering property.

A coating density of the aliphatic carboxylic acid coating the surfaceof the copper particles was determined by the method described below.

According to the results of the Tof-SIMS analysis, when it is assumedthat the whole of the heating loss component is lauric acid, the numberof lauric acid molecules contained in the coated copper particles isrepresented by the formula (3).

[The number of lauric acid molecules]=M _(acid)/(M _(W) /N _(A))  (3)

In the above formula, M_(acid) is a value of mass (g) of the heatingloss measured, M_(W) is the molecular weight of lauric acid (g/mol), andN_(A) is an Avogadro's constant (6.02×10²³/mol).

When it is assumed that the primary particle diameter determined by anexamination under a SEM is substantially derived from reduced copper andthe shape of the particle is a sphere, the number of the copperparticles per 1 g is represented by the formula (4).

[The number of copper particles per 1 g]=M _(Cu)/[(4πr³/3)×d×10^(−21])  (4)

In the above formula, M_(Cu) is a value of mass (g) calculated from thevalue of a heating loss measured, r is a radius (nm) for the primaryparticle diameter calculated from an examination under a SEM, and d is adensity (the density of copper was substituted for d; d=8.94). Aparticle surface area of the copper particles per 1 g is represented bythe formula (5) using the formula (4).

[Copper particle surface area (nm²) per 1 g]=[The number of copperparticles per 1 g]×4πr ²  (5)

A coating density of lauric acid (molecules/nm²) in the copper particlesis represented by the formula (6) using the formulae (3) and (5).

[Coating density]=[The number of lauric acid molecules]/[Copper particlesurface area per 1 g]   (6)

From the results of the calculation, the coating density of lauric acidin the coated copper particles was found to be 4.23 molecules/nm².

According to “Chemistry and Education, Vol. 40, No. 2 (1992),Determining cross-sectional area of stearic acid molecule, -Experimentalvalues and Calculated values-”, the minimum area is calculated from theVan der waals radius of a stearic acid molecule, and the theoreticalvalue of saturated coating area determined from the above calculatedvalue is about 5.00 molecules/nm². From the theoretical value, it ispresumed that the coated copper particle according to the presentembodiment has lauric acid localized on the surface of the particle at arelatively high density. The reason that the coated copper particleshave excellent oxidation resistance even though coating with lauric acidis made through physical adsorption weaker than chemical adsorption isconsidered to reside in the effect of coating with lauric acid at a highdensity.

Next, taking Example 1 as an example, the reaction mechanism in thepresent invention was presumed by making an analysis for components ofthe gas and distillate discharged during the reaction.

<Analysis for Gas Components>

Method: Gas chromatographyMeasurement apparatus: GL Science GL320Detector: Thermal conductivity detector (TCD)Column: Stainless steel column φ3 mm×2 mColumn packing material (hydrogen): Molecular Sieve 5AColumn packing material (carbon dioxide): Active CarbonCarrier gas (hydrogen): N₂, 20 mL/minCarrier gas (carbon dioxide): He, 50 mL/minTemperature for measurement: 43 to 50° C.Current value: 70 to 120 mA

<Analysis for Distillate Components>

Method: Infrared spectroscopyMeasurement apparatus: PerkinElmer Spectrum One

TABLE 1 Sample No. Sampling time (min) Discharge rate (L/min) H₂ (%) CO(%) CO₂ (%) Remarks (1) 17 0.22 <0.1 <0.1 0.5 Remainder is N₂ (2) 191.03 0.5 <0.1 52.3 Remainder is N₂ (3) 26 2.45 0.5 <0.1 88.5 Remainderis N₂ (4) 28 2.28 12.0 <0.1 81.0 (5) 31 2.09 45.1 <0.1 50.0 (6) 44 1.4552.9 <0.1 45.9 (7) 68 0.88 36.5 <0.1 57.9 (8) 76 0.83 34.4 <0.1 60.1 (9)99 0.42 33.2 <0.1 64.6 (10)  119 0.27 29.3 <0.1 62.6

At the initial stage of the reaction, from the fact that the dischargedgas component is carbon dioxide gas and the reaction temperature isabout 120° C. (FIG. 4), it is considered that a reaction of reactionformula 1 shown below occurs, and then a reaction proceeds according tothe reaction mechanism of reaction formula 2. That is, one molecule oflauric acid first causes an equilibrium exchange reaction with onemolecule of formic acid in a copper formate-amino alcohol complex.

(HCOO⁻)(HCOO⁻)Cu²⁺.(H₂NC₃H₆OH)₂+C₁₁H₂₃COOH→(C₁₁H₂₃COO⁻)(HCOO⁻)Cu²⁺.(H₂NC₃H₆OH)₂+HCOOH  (Reactionformula 1)

Generally, it is considered that copper formate, which exhibits thermaldecomposition properties at about 210 to 250° C., forms a complexcompound together with lauric acid and 3-amino-1-propanol to lower thethermal decomposition temperature. While the complex compound undergoesa thermal decomposition reaction at about 100 to 130° C. to releasecarbon dioxide gas, a reduction reaction of bivalent copper ionsproceeds (reaction formula 2). It is considered that lauric acidphysically adsorbs on reduced copper formed after decomposition of thecomplex compound.

(C₁₁H₂₃COO)(HCOO⁻)Cu²⁺.(H₂NC₃H₆OH)₂→Cu:C₁₁H₂₃COOH+2H₂NC₃H₆OH+CO₂  (Reactionformula 2)

It is considered that lauric acid adsorbing on the reduced copperundergoes a reversible equilibrium (reaction formula 3), and a lauricacid equilibrium exchange reaction with a copper formate-alkanol aminecomplex is caused again in the vicinity of the reduced copper accordingto reaction formula 1, so that reduced metal nuclei are successivelygenerated.

At the last stage of the reaction, the discharged gas components arehydrogen gas and carbon dioxide gas, and, from the ratio of the gascomponents, the reaction mechanism shown below is considered to proceed.

(HCOO⁻)(HCOO⁻)Cu²⁺.(H₂NC₃H₆OH)₂→Cu+2H₂NC₃H₆OH+H₂+2CO₂  (Reaction formula4)

It is considered that, as lauric acid is consumed as a coating materialin the particle growth mechanism for the reduced copper rather than inthe equilibrium exchange reaction for forming a complex compound withcopper formate, not only the reaction of reaction formula 1 but also thereaction of reaction formula 4 proceed at the same time.

The distillate distilled during the reaction was water molecules (FIG.5).

Japanese Unexamined Patent Publication No. 2011-032558 discloses thatthe remaining water molecules cause the copper formate-amino alcoholcomplex to suffer hydrolysis to form copper oxide. When this is appliedto the present invention, copper oxide is formed according to thereaction shown below.

(HCOO⁻)(HCOO⁻)Cu²⁺.(H₂NC₃H₆OH)₂+H₂O→CuO+2H₂NC₃H₆OH+2HCOOH  (Reactionformula 5)

When water molecules are present in the reaction system, the mechanismof reaction formula 5 is considered to proceed to form copper oxide.However, the presence of copper oxide was not confirmed in the coatedcopper particles obtained in Example 1 (FIG. 1A). From these results, itis presumed that a reaction mechanism for reducing copper oxide separatefrom the primary reaction is present. As a reduction reaction mechanismfor copper oxide, a reduction reaction by formic acid, such as reactionformula 6, is considered.

2CuO+2HCOOH→Cu₂O+HCOOH+H₂O+CO₂→2Cu+2H₂O+2CO₂  (Reaction formula 6)

Alternatively, it is considered that copper oxide and formic acidregenerate copper formate as shown in reaction formula 7, so that areduction reaction proceeds.

CuO+2HCOOH→(HCOO⁻)(HCOO⁻)Cu²⁺+H₂O  (Reaction formula 7)

The regenerated copper formate undergoes a reaction of reaction formula1, and a reaction is caused according to reaction formula 2, so that areduction reaction proceeds. It is considered that water molecules areformed due to the side reaction and discharged as a distillate. Thisside reaction is a reaction that can be caused when formic acid ispresent in the reaction system. Therefore, it is presumed that even whencopper oxide is unexpectedly formed, the reduction reaction mechanismaccording to reaction formulae 6 and 7 enables a synthesis of coatedcopper particles having no oxide film.

For checking whether the above-mentioned reduction reaction mechanismcan be caused in the method according to the present embodiment, areaction was conducted under conditions such that copper oxide isintentionally added to the reaction system. The results of the reactionare described below in Reference Example 2.

Reference Example 2

A 100 mL four-neck glass flask equipped with a stirrer, a thermometer, areflux condenser, and a nitrogen introducing tube was set in an oilbath. 12.0 g (0.08 mol) of copper formate anhydride, 2.0 g of copperoxide (manufactured by Kanto Chemical Co., Inc.) (0.32 equivalent/copperformate anhydride), 2.0 g of lauric acid (manufactured by Kanto ChemicalCo., Inc.; 1st grade reagent) (0.12 equivalent/copper formateanhydride), 4.4 g of tripropylene glycol monomethyl ether (manufacturedby Tokyo Chemical Industry Co., Ltd.) (0.27 equivalent/copper formateanhydride), and 16.6 g of SWACLEAN 150 (manufactured by Godo Co., Ltd.)(1.67 equivalent/copper formate anhydride) were placed in the flask, andmixed with one another while stirring at 200 rpm. The resultant mixturewas heated in a nitrogen gas atmosphere while stirring at 200 rpm untilthe temperature of the mixture became 50° C. To the mixture was addeddropwise slowly 21.0 g of 3-amino-1-propanol (manufactured by TokyoChemical Industry Co., Ltd.) (3.50 equivalents/copper formateanhydride). After completion of the addition, the resultant mixture washeated while stirring at 340 rpm until the temperature of the mixturebecame about 120° C. As the temperature increased, the reaction mixturestarted changing in color from deep blue to brown and bubbles of carbondioxide gas were generated. A time when the generation of bubbles ofcarbon dioxide gas was ended was determined as a reaction end point, andthe temperature control using an oil bath was stopped, so that thereaction mixture was cooled to room temperature. After cooling to roomtemperature, 20.0 g of methanol (manufactured by Kanto Chemical Co.,Inc.) was added to and mixed with the reaction mixture. The resultantmixture was allowed to stand for 30 minutes or longer, and the resultantsupernatant was removed by decantation to obtain a precipitate. To theprecipitate were added 20.0 g of methanol (manufactured by KantoChemical Co., Inc.) and 10.0 g of acetone (manufactured by KantoChemical Co., Inc.), followed by mixing with one another. The resultantmixture was allowed to stand for 30 minutes or longer, and the resultantsupernatant was removed by decantation to obtain a precipitate. Thisoperation was repeated once. The resultant precipitate was transferredto a 100 mL eggplant-shaped flask while washing the precipitate using20.0 g of methanol (manufactured by Kanto Chemical Co., Inc.). Afterallowed to stand for 30 minutes or longer, the resultant supernatant wasremoved by decantation, and the obtained precipitate was set to a rotaryevaporator and subjected to vacuum drying at 40° C. under 1 kPa or less.After completion of the vacuum drying, the precipitate was cooled toroom temperature, and the reduced pressure was increased by introducingnitrogen gas, obtaining 6.5 g of a brown copper powder.

With respect to the coated copper particles obtained in ReferenceExample 2, a powder X-ray analysis was conducted (FIG. 6). It is foundthat almost all the copper oxide added has been reduced to be convertedto reduced copper. The result suggests that even when copper oxide isformed in the reaction system, the copper oxide is converted to reducedcopper by a reduction reaction due to formic acid.

According to the example of reaction mechanism described in JapaneseUnexamined Patent Publication No. 2011-032558, like the presentembodiment, a copper formate-amino alcohol complex containing analiphatic carboxylic acid is formed.

(R₁COO⁻)(HCOO⁻)Cu²⁺:[(C₂H₅)₂NC₂H₄OH]₂→Cu:[(C2H₅)₂NC₂H₄OH]₂+R₁COOH+CO₂  (Reactionformula 8)

Further, Japanese Unexamined Patent Publication No. 2011-032558 hasdescriptions that the amino alcohol is limited to abidentate-coordinating amino alcohol and hence the copper formatecomplex having such an amino alcohol has a low thermal decompositiontemperature, and that formic acid eliminated due to an exchange reactionof the aliphatic carboxylic acid and the copper formate-amino alcoholcomplex is discharged out of the system. Furthermore, the reactionconditions at low temperatures are employed, and therefore it isconsidered that a reduction reaction caused by formic acid present inthe reaction system does not proceed.

Further, according to the example of reaction mechanism described inJapanese Unexamined Patent Publication No. 2008-013466, all the formicacid present in the reaction system is decomposed into hydrogen andcarbon dioxide gas by a thermal decomposition reaction of the complexcompound.

(HCOO⁻)(HCOO⁻)Cu²⁺.(NH₂R₂)₂→Cu+2NH₂R₂+H₂+2CO₂  (Reaction formula 9)

In contrast, the reaction mechanism in the present embodiment has boththe formation of reduced copper due to a thermal decomposition reactionof the copper formate complex and the reaction mechanism in which theby-produced copper oxide is reduced according to reaction formulae 6 and7, and therefore the coated copper particles synthesized by the methodaccording to the present embodiment are unlikely to be oxidized. Themethod according to the present embodiment does not need strictproduction control in respect of water and oxygen which are mentioned asthe cause of oxidation of metal copper, and thus is a method suitablefor more easily synthesizing coated copper particles.

Next, with respect to the coated copper particles obtained in Examples 1to 4 and Comparative Examples 1 to 5, a calculation of an SP value forthe raw materials, a TG-DTA measurement, a powder X-ray analysis (XRD),and a SEM examination were performed. The methods for the measurementare as described below. The results are shown in Table 2.

TABLE 2 Com- Com- Com- Com- Com- parative parative parative parativeparative Example 1 Example 2 Example 3 Example 4 Example 1 Example 2Example 3 Example 4 Example 5 Amino alcohol 13.45 12.74 12.33 12.74 8.489.98 10.78 12.33 13.45 SP value Solvent 8.21 8.21 7.53 7.53 8.21 8.218.21 8.21 9.77 SP value ΔSP value 5.24 4.53 4.80 5.21 0.27 1.77 2.574.12 3.68 Organic component 1.80 8.68 7.03 3.71 2.69 0.73 11.29 8.900.31 amount (wt %) XRD crystal particle 48.9 23.8 15.6 34.1 43.5 147.779.0 8.8 166.0 diameter (nm) SEM primary particle 85.8 51.2 58.8 50.5137.7 626.2 2678.6 1125.5 813.9 diameter (nm) Crystallinity 0.57 0.460.26 0.65 0.32 0.24 0.03 0.01 0.20 Form of particles NanoparticlesNanoparticles Nanoparticles Nanoparticles Aggregate Aggregate AggregateAggregate Aggregate form form form form form

As can be seen from the results shown in Table 2, when a differencebetween the SP value of the amino alcohol constituting the copperformate-amino alcohol complex and the SP value of the reaction solventor the mixed solvent of a prime solvent and a co-solvent, i.e., a ΔSPvalue is as large as 4.2 or more, coated copper particles which areseparate from one another can be produced. However, when a ΔSP value is4.2 or less, the produced coated copper particles are not separate fromone another but are in an aggregate form, and hence the method in thiscase is not suitable as a method for producing coated copper particles.

Further, when the proportion of the size of crystal particle to that ofprimary particle is defined as crystallinity, the crystallinity is 0.25or more, and is about 0.50 in most cases, which indicates that a singlelarge crystal particle constitutes the coated copper particle.

With respect to the coated copper particles obtained in Examples 5 to 8and Comparative Examples 6 and 7, a powder X-ray analysis and a SEMexamination were performed. The results are shown in Table 3.

TABLE 3 Comparative Comparative Example 1 Example 5 Example 6 Example 7Example 8 Example 9 Example 6 Example 7 Oleic acid — 68.1 — — — — — —Lauric acid 68.1 — 16.0 144.0 68.1 — — — Octanoic acid — — — — — 45.4 —— XRD crystal particle 48.9 33.7 39.0 48.8 45.4 45.9 56.0 76.0 diameter(nm) SEM primary particle 85.8 70.2 75.8 92.0 89.9 104.7 167.7 211.4diameter (nm) Standard deviation 13.2 11.0 17.4 11.1 17.4 41.5 59.9 60.4Crystallinity 0.57 0.48 0.51 0.53 0.51 0.44 0.33 0.36 Variation 0.150.16 0.23 0.12 0.19 0.40 0.36 0.29

When the temperature increase rate is constant during the reaction, thereduction reaction rate is the same, and hence the amount of the metalnuclei generated is the same. From this, it is apparent that theparticle diameter and particle size distribution vary depending on thefactors controlling the size and stability of the micro-reaction fieldwhich is like a water-in-oil emulsion. When it is likely that themicro-reaction field is stabilized, the particle diameter and thevariation tend to be smaller, and this tendency is remarkable as thecarbon chain of the aliphatic carboxylic acid becomes longer (acomparison between Examples 1, 5, and 9). Further, when the molar numberof the aliphatic carboxylic acid relative to copper atoms is increased,the reaction field tends to be stabilized, but, when the viscosity ofthe reaction system is increased, the particle diameter is increasedeven though the variation is small (a comparison between Examples 1, 6,and 7).

It has been confirmed that even when the ratio of the prime solvent andthe co-solvent in the reaction solvent is changed so that the average SPvalue varies from 8.21 to 8.90, no large change is caused in theproperties of the obtained coated copper particles as long as the ΔSPwhich is a difference in SP value between the amino alcohol and thereaction solvent is surely the value as defined in the present invention(a comparison between Examples 1 and 8).

In this connection, in accordance with the working Example of JapaneseUnexamined Patent Publication No. 2013-047365, which is similar to thecoated copper particle of the present embodiment in the point that thecoating material is an aliphatic carboxylic acid, the production ofcoated copper particles was attempted in Comparative Examples 6 and 7.These methods are a method for producing coated copper particles, inwhich a reservoir which is an insoluble solid is used as a source ofsupplying copper atoms. Copper(I) oxide having a particle diameter assmall as 2 to 4μ was used as a raw material, but the unreacted materialremained after a predetermined reaction time. When the reaction time wasincreased twice, the copper oxide was stoichiometrically converted toreduced copper, but the average particle diameter of the resultantparticles was increased. Further, the variation of the particle sizedistribution of the resultant particles was as large as 0.38 or 0.29.

Test Example 1 Evaluation of an Oxidation Resistance

With respect to the coated copper particles produced under theconditions in Example 1, a powder X-ray analysis was conducted for thoseimmediately after produced and those after stored at 25° C. for 4 monthsto check whether oxidation proceeded or not (FIGS. 1A and 1B). Evenafter 4 months, no oxide component was detected, and the result hasconfirmed that the coated copper particles produced by the methodaccording to the present embodiment have excellent oxidation resistance.

Test Example 2

Properties of a Calcined Film Formed from a Coated Copper Particle Paste

The coated copper particles produced under the conditions in Example 1were dispersed in a solvent to prepare a copper paste composition, andthe paste composition was calcined in a nitrogen gas atmosphere at 300°C. or 350° C. for one hour to form a copper film (copper paste sinteredlayer), and an electric resistance of the formed film was measured. Thecoated copper particles and a solvent having the composition shown belowwere dispersed and kneaded using a mortar so as to form a paste,preparing a copper paste composition having a metal content of 33% byvolume.

Copper paste composition A Coated copper particles in Example 1 10 Partsby mass KYOWANOL M (manufactured by NH Neochem 2 Parts by mass Co.,Ltd.)

The above-prepared copper paste was applied to a polyimide film having athickness of 40 μm and having a 12 μm copper foil laminated on the backside so that the wet thickness of the applied paste became about 10 μm,and dried and calcined in a nitrogen gas atmosphere to prepare a samplefor evaluation.

An average thickness of the film of calcined copper was measured byexamining the cross-section of the film under a SEM. The thickness ofthe copper paste sintered layer was found to be 2.4 μm.

SEM examination images of the prepared coated copper particles andcalcined copper film are shown in FIGS. 24A and 24B, a SEM cross-sectionexamination image of the sample for evaluation used in the measurementis shown in FIG. 25, and the results of the measurement are shown inTable 4.

In FIG. 25, copper paste sintered layer 10 which is a calcinationproduct of the copper paste composition is formed on the surface ofpolyimide film layer 20 on the other side of copper foil layer 30,wherein polyimide film layer 20 has a thickness of 40 μm, and has copperfoil layer 30 having a thickness of 12 μm on one side of the polyimidefilm.

Test Example 3

Using the coated copper particles produced in Comparative Example 7,copper paste composition B was prepared in the same manner as in TestExample 2, and calcined in a nitrogen gas atmosphere at 350° C. for onehour to form a copper film, and an electric resistance of the formedfilm was measured.

Copper paste composition B Coated copper particles in ComparativeExample 7 10 Parts by mass KYOWANOL M (manufactured by NH Neochem 2Parts by mass Co., Ltd.)

SEM examination images of the prepared coated copper particles andcalcined copper film are shown in FIGS. 26A and 26B, and the results ofthe measurement of electric resistance are shown in Table 4.

Test Example 4

Properties of a Calcined Film Formed from a Mixed Paste of the CoatedCopper Particles and a Copper Powder

The coated copper particles produced in Example 1 were added as asintering agent to a commercially available copper powder to prepare acopper paste composition, and the paste composition was calcined in anitrogen gas atmosphere at 300° C. or 350° C. for one hour to form acopper film, and an electric resistance of the formed film was measured.The materials having the composition shown below were dispersed andkneaded by means of a three-roll mill to prepare a copper pastecomposition having a metal content of 60% by volume.

Copper paste composition C Coated copper particles in Example 1 100Parts by mass Wet-process copper powder 2.0 μm (1200N, 100 Parts by massmanufactured by Mitsui Mining & Smelting Co., Ltd.) Wet-process copperpowder 0.8 μm (1050Y, 25 Parts by mass manufactured by Mitsui Mining &Smelting Co., Ltd.) Polyacrylic acid dispersant 0.5 Part by massKYOWANOL M (manufactured by NH Neochem 15 Parts by mass Co., Ltd.)

The above-prepared copper paste was applied to a polyimide film having athickness of 40 μm and having a 12 μm copper foil laminated on the backside so that the wet thickness of the applied paste became about 10 μm,and dried and calcined in a nitrogen gas atmosphere to prepare a samplefor evaluation.

An average thickness of the film of calcined copper was measured byexamining the cross-section of the film under a SEM. The thickness ofthe copper paste sintered layer was found to be 4.2 μm.

A SEM cross-section examination image of the sample for evaluation usedin the measurement is shown in FIG. 27, and the results of themeasurement are shown in Table 4.

In FIG. 27, copper paste sintered layer 10 which is a calcinationproduct of the copper paste composition is formed on the surface ofpolyimide film layer 20 on the other side of copper foil layer 30,wherein polyimide film layer 20 has a thickness of 40 μm, and has copperfoil layer 30 having a thickness of 12 μm on one side of the polyimidefilm.

TABLE 4 Calcination conditions (in nitrogen atmosphere) DryingPre-calcination Main calcination Volume resistivity conditionsconditions conditions (μΩ · cm) Copper paste 120° C. × 15 min 270° C. ×30 min 300° C. × 1 hr 6.0 composition A 120° C. × 15 min 270° C. × 30min 350° C. × 1 hr 5.3 Copper paste 120° C. × 15 min 270° C. × 30 min350° C. × 1 hr 22.3 composition B Copper paste 120° C. × 15 min 270° C.× 30 min 300° C. × 1 hr 5.1 composition C 120° C. × 15 min 270° C. × 30min 350° C. × 1 hr 4.4

A copper paste composition was prepared using the coated copperparticles capable of being calcined in a nonreducing atmosphere, and asintered copper film formed from the composition was evaluated withrespect to the electric properties and the surface compositionstructure, and thus industrial technical usefulness of the film wasconfirmed.

The electric properties and the surface composition structure asmeasured by XPS were evaluated with respect to each of copper pastecomposition A using the coated copper particles produced in Example 1(Test Example 2), copper paste composition C having a commerciallyavailable copper powder mixed into the coated copper particles producedin Example 1 (Test Example 4), and copper paste composition B forcomparison using the coated copper particles prepared in ComparativeExample 7 in accordance with Example 1 of Japanese Unexamined PatentPublication No. 2013-047365 (Test Example 3).

As a result, copper paste compositions A and C prepared using the coatedcopper particles in Example 1 exhibited conductivity performance as highas 4 to 6 μΩ*cm. Further, the results have confirmed that copper pastecompositions A and C exhibit such high adhesion to an untreatedpolyimide surface that the composition is not peeled off the surface ina general bending test. Copper paste composition B, which uses thecoated copper particles similarly coated with an aliphatic carboxylicacid, exhibited a volume resistivity value of 22 μΩ*cm which is higherthan that of copper bulk by one digit or more.

With respect to the surface composition structure of the sintered copperfilms formed from copper paste compositions A and B, a compositionalanalysis by XPS was conducted, and observations were made on themechanism in which a difference is caused in the electric propertiesbetween the surface compositions.

The data in FIGS. 28 and 29 obtained by Narrow Scan show information ofthe compositions of the uppermost surfaces of the sintered copper filmsformed from copper paste compositions A and B. It is found that theuppermost surface of the sintered copper film formed from copper pastecomposition B has a high reduced copper ratio and has a small copperoxide component ratio and a small organic component ratio, as comparedto that of copper paste composition A. The uppermost surface of thesintered copper film formed from copper paste composition A is coatedwith an aliphatic carboxylic acid which is presumed to be lauric acid,and the coating aliphatic carboxylic acid causes the organic componentratio to be increased. The film has an oxide component, but has reducedcopper which is exposed in a considerable amount, and therefore the filmis presumed not to be reduced in the contact resistance.

Next, the film was etched with argon ions and the surface composition ina region several nm from the uppermost surface of the film was clarifiedusing a Depth Profile analysis by XPS.

From the results of the analysis, with respect to both copper pastecompositions A and B, it is found that the region at a depth of morethan 1 to 2 nm from the uppermost surface is comprised of reducedcopper. With respect to copper paste composition B, even in a region ata depth of more than several nm, the carbon content is not reduced, ascompared to that of copper paste composition A. From this, the sinteringdensity for copper paste composition B is expected to be low.

Further, the results of the SEM examination clearly show a difference inthe sintering density between the paste compositions, and this isconsidered to appear as a difference in the electric properties betweenthe paste compositions. Accordingly, it is found that an important thingfor the mechanism in which the coated copper particles exhibit alow-temperature sintering property is not that the coating material forthe coated copper particles is removed at low temperatures but is thatthe coating material is removed to an appropriate extent such thatcontact of the copper particles with each other, necking, andinterdiffusion of copper atoms are not inhibited.

In the sintered copper film obtained from the coated copper particle ofthe present invention, the aliphatic carboxylic acid as a coatingmaterial, which has been present between the particles, is efficientlyremoved and, meanwhile, contact of the particles with each other,necking, and interdiffusion of copper atoms are achieved, so thatproperties close to the resistance value of copper bulk can be achieved.Further, it also has been found that consequently, the uppermost surfaceof the sintered copper film is coated with the aliphatic carboxylic acidas a coating material, which can be expected to serve as a barrier layerfor oxygen.

The method for producing coated copper particles of the presentinvention can produce coated copper particles having excellent oxidationresistance, which have a particle diameter controlled due to a uniquemicro-reaction field, and which have the surface thereof coated with ahigh-density coating layer of an aliphatic carboxylic acid. Further, itis considered that the obtained coated copper particles are comprisedalmost mainly of reduced copper and have the aliphatic carboxylic acidas a coating material bonded thereto through physical adsorption. Forthis reason, the coating material is eliminated at a temperature closeto the boiling point of the coating material so that the coated copperparticles exhibit a sintering ability. Therefore, a copper sintered filmhaving a volume resistivity value close to that of copper bulk can beobtained from the coated copper particles, for example, underatmospheric pressure at a temperature as relatively low as 300° C. orlower. Further, a paste of the coated copper particles is preparedusing, for example, a solvent, and a wiring pattern can be formed fromthe prepared paste by a method, such as screen printing, and the formedwiring pattern can be calcined in an atmosphere of a low oxygenconcentration state, enabling construction of a production processhaving such high general-purpose properties that, for example, a generalnitrogen-purged furnace can be used.

The whole of the disclosure of Japanese Patent Application No.20104-112794 (application date: May 30, 2014) is included in the presentspecification by making a reference to it.

All the reference documents, patent applications, and technicalstandards described in the present specification are included in thepresent specification by making a reference to them to the same extentas that in the case where each of the reference documents, patentapplications, and technical standards is specifically and individuallyshown to be included in the present specification by making a referenceto each of them.

1.-6. (canceled)
 7. A coated copper particle containing a copperparticle coated with an aliphatic carboxylic acid having an aliphaticgroup having 5 or more carbon atoms, wherein the coated copper particlehas an average primary particle diameter D_(SEM) of 0.02 to 0.2 mm, asdetermined by a SEM examination, wherein a value of a coefficient ofvariation of the particle size distribution (standard deviationSD/average primary particle diameter DSEM) is 0.1 to 0.5, and the coatedcopper particle has a DXRD/DSEM ratio of 0.25 to 1.00 wherein theDXRD/DSEM ratio is a ratio of a crystal particle diameter DXRD, asdetermined by a powder X-ray analysis, to an average primary particlediameter DSEM, as determined by a SEM examination.
 8. A coated copperparticle having a surface coated with an aliphatic carboxylic acidhaving an aliphatic group having 5 or more carbon atoms, wherein thecoated copper particle has a DXRD/DSEM ratio of 0.25 to 1.00 wherein theDXRD/DSEM ratio is a ratio of a crystal particle diameter DXRD, asdetermined by a powder X-ray analysis, to an average primary particlediameter DSEM, as determined by a SEM examination, and wherein thecoated copper particle is a thermal decomposition product of a complexcompound formed in a reaction mixture containing copper formate, anamino alcohol, an aliphatic carboxylic acid having an aliphatic grouphaving 5 or more carbon atoms, and a solvent, wherein a DSP value, whichis a difference in SP value between the amino alcohol and the solvent,is 4.2 or more. 9.-11. (canceled)
 12. The coated copper particleaccording to claim 7 which is a thermal decomposition product of acomplex compound formed in a reaction mixture containing copper formate,an amino alcohol, an aliphatic carboxylic acid having an aliphatic grouphaving 5 or more carbon atoms, and a solvent, wherein a DSP value, whichis a difference in SP value between the amino alcohol and the solvent,is 4.2 or more.
 13. The coated copper particles according to claim 7,wherein the aliphatic carboxylic acid physically adsorbs on the surfaceof the copper particles.
 14. The coated copper particles according toclaim 8, wherein the aliphatic carboxylic acid physically adsorbs on thesurface of the copper particles.
 15. The coated copper particlesaccording to claim 12, wherein the aliphatic carboxylic acid physicallyadsorbs on the surface of the copper particles.
 16. A conductivecomposition comprising coated copper particles according to claim 7 anda medium.
 17. A conductive composition comprising coated copperparticles according to claim 8 and a medium.
 18. A conductivecomposition comprising coated copper particles according to claim 12 anda medium.
 19. A conductive composition comprising coated copperparticles according to claim 13 and a medium.
 20. A conductivecomposition comprising coated copper particles according to claim 14 anda medium.
 21. A conductive composition comprising coated copperparticles according to claim 15 and a medium.
 22. A circuit formedarticle comprising a substrate, and a wiring pattern which is disposedon the substrate, and which is a thermal treatment product of theconductive composition according to claim
 16. 23. A circuit formedarticle comprising a substrate, and a wiring pattern which is disposedon the substrate, and which is a thermal treatment product of theconductive composition according to claim
 17. 24. A circuit formedarticle comprising a substrate, and a wiring pattern which is disposedon the substrate, and which is a thermal treatment product of theconductive composition according to claim
 18. 25. A circuit formedarticle comprising a substrate, and a wiring pattern which is disposedon the substrate, and which is a thermal treatment product of theconductive composition according to claim
 19. 26. A compositioncomprising the coated copper particles according to claim 7, aminoalcohol, and a solvent, wherein a DSP value, which is a difference in SPvalue between the amino alcohol and the solvent, is 4.2 or more.
 27. Acomposition comprising the coated copper particles according to claim 8,amino alcohol, and a solvent, wherein a DSP value, which is a differencein SP value between the amino alcohol and the solvent, is 4.2 or more.28. A composition comprising the coated copper particles according toclaim 12, amino alcohol, and a solvent, wherein a DSP value, which is adifference in SP value between the amino alcohol and the solvent, is 4.2or more.
 29. A composition comprising the coated copper particlesaccording to claim 13, amino alcohol, and a solvent, wherein a DSPvalue, which is a difference in SP value between the amino alcohol andthe solvent, is 4.2 or more.